int curcolor = 0; //0 is red, 1 is green, 2 is blue
int r = 255;
int g = 255;
int b = 255;

void setup() {
  size(100, 100);
  frameRate(5);
}

void draw() {
  background(color(r,g,b));
}

void keyPressed() {
  if(curcolor == 0) {
    r = (int)key;
  } else if(curcolor == 1) {
    g = (int)key;
  } else {
    b = (int)key;
  }
  curcolor = (curcolor+1)%3;
}